1. WWW::Mechanize::Page#images.rb

require 'mechanize'

WWW::Mechanize::Page.class_eval do
  def images
    @images ||= search('img').map do |img|
      case src = img['src']
      when %r{^https?://}
        src
      else
        (uri + src).to_s
      end
    end
  end
end

2. example.rb

irb(main):001:0> include ::Webrat::Methods
irb(main):002:0> Webrat.configure.mode = :mechanize
irb(main):003:0> visit 'http://gist.github.com/'
irb(main):004:0> webrat.response.images
=> ["http://gist.github.com/images/modules/header/logo_gist.png", ...